(1) Field of the Invention
The present invention relates to a digital signal reconstruction circuit which can accurately reconstruct a digital signal recorded on a recording medium such as magnetic tape. More particularly, it relates to a digital signal reconstruction circuit which reads a digital signal recorded on a recording medium at a plurality of sampling points. The logical product or logical sum of the signal samples at the sampling points is selected as the preceding logical data signal of "1" or "0", respectively and is regarded as the content of present-time data.
(2) Description of the Prior Art
Recently, in a PCM (pulse-coder modulation) recorder or the like, a high density digital signal is recorded and played back. With such high-density recording, when the frequency band of the recording and playback system, including the magnetic tape, magnetic heads, etc. cannot be sufficiently widened, the quality of the playback signal is degraded. The degradation is caused by the intersymbol interference between recording bit patterns. As a result it is difficult to extract the digital signal, which difficulty leads to code errors.
In a household PCM encoder/decoder, an audio signal is recorded and played back utilizing a household video cassette system or a part thereof. In such a system, television synchronizing pulses are recorded in a manner such that they appear with original audio signal. As a result the part of the signal occurring immediately after the synchronizing pulse may experience intersymbol interference. Moreover, the recording density of the household video cassette system has become so high in recent years that it is impossible to obtain the necessary frequency bandwidth. This causes an increase in the jitters of the recording and playback systems. It has accordingly become very difficult to precisely extract recorded data.
When demodulating such signal the data have heretofore been extracted at the central positions of the respective bit cells 1 of a recorded waveform as illustrated in FIGS. 1(a), 1(b) and 1(c), wherein the abscissas represents time and the ordinate amplitude. More specifically, when a playback signal 2 as shown in FIG. 1(a) is output from a tape recorder or the like, it is compared with a predetermined threshold level 3 and then wave-shaped by means of a comparator to obtain the wave-shaped output 4 as shown in FIG. 1(b). The wave-shaped output 4 is latched by latch pulses 5 shown in FIG. 1(c). These latch pulses are obtained by frequency dividing and phase adjusting the horizontal synchronizing signal or vertical synchronizing signal interleaved with the data so that they rise at the centers of the bit cells 1 in synchronism with the respective data. With such a method, the signal 2 may be degraded and deformed by the intersymbol interference as illustrated in FIG. 1(d). More specifically, when the wave-shaped output 4 is latched by the latch signal 5 (FIG. 1(f)), erroneous data is extracted as shown at 4a in FIG. 1(e). When the data have been recorded in a high-density recording mode with the household video cassette system, the peak position of the playback data becomes low at points at which the recording wavelength is small relative to the frequency band of the recording system. For example, the digital signal bits forming the playback signal occurring immediately after synchronizing pulses undergo the intersymbol interference and degradation shown in FIG. 1(d). Likewise, a "0" signal bit appearing between signal bits of successive "1's" or conversely a " 1" signal bit appearing between signal bits of successive "0's" may undergo intersymbol interference and degradation.
Accordingly, in PCM recording and playback of, e.g., the audio signal with the household video cassette system, it has been necessary to correct the data which are longer or shorter than one bit cell or have been positionally shifted on account of intersymbol interference. This correction has not been easy.